The present invention relates to a hydrodynamic type porous oil-impregnated bearing being impregnated with lubricating oil or lubricating grease in a bearing body of porous substance, such as sintered metal, to have a self-lubricating function, supporting a slide surface of a shaft in a non-contact manner by a lubricating oil film produced in a bearing clearance due to hydrodynamic function of hydrodynamic pressure generating grooves in a bearing surface. The bearing of the invention is suitable for use particularly in machines and instruments of which high rotation accuracy at high speed is required, such as spindle motors for polygon mirror of laser beam printer (LBP), magnetic disk drives (HDDs), or the like, and in machines and instruments which are driven at high speed with a large imbalance load produced in that a disk is mounted thereon, such as spindle motors for DVD-ROM, or the like.
In such small-sized spindle motors associated with information-handling devices, improved rotation performance and cost reduction are required, as a means therefor, possibility of changing bearings for the spindle from a rolling bearing to a porous oil-impregnated bearing has been investigated. However, since a porous oil-impregnated bearing is a kind of cylindrical bearing, it tends to produce unstable vibrations where the shaft eccentricity is small, inducing the so-called whirl in which the shaft is subjected to a revolving vibration at a rate which is half the rotary speed. Accordingly, it has heretofore been attempted to form hydrodynamic pressure generating grooves, such as the herringbone or spiral shape, in bearing surface, so as to produce a lubricating oil film in a bearing clearance by the function of the hydrodynamic pressure generating grooves which accompanies the rotation of the shaft, to thereby support the shaft in a noncontact manner (hydrodynamic type porous oil-impregnated bearing).
A porous oil-impregnated bearing being formed hydrodynamic pressure generating grooves in a bearing surface is disclosed in Japanese Utility Model Koukoku Shouwa 63-19627. In this prior art, a region of the hydrodynamic pressure generating grooves in the bearing surface is worked to seal surface openings thereon. Such construction, however, has the following drawback. Since the surface openings on the region of the hydrodynamic pressure generating grooves completely sealed, the circulation of oil, which is the greatest feature of the porous oil-impregnated bearing, is obstructed. Therefore, the oil which has been exuded in the bearing clearance is forced into the bent portions of the groove region by the action of the hydrodynamic pressure generating grooves and stays there. A great shearing action is present in the bearing clearance, and this shearing force and frictional heat cause the oil staying in the groove region to be denatured, while a rise in temperature tends to accelerate oxidative deterioration of the oil. Therefore, the bearing life is shortened. On the other hand, besides plastic processing, it has been proposed to employ coating or the like as another means for applying a surface treatment, however, it is necessary that the thickness of such coating film be less than the groove depth, and it is very difficult to apply a coating film which is some .mu.m thick solely to the groove region.
In order to secure the rotation accuracy of the shaft, a plurality of bearings, e.g., two bearings, are usually used. Further, bearings are used mostly by being pressed into a housing. Thus, to secure a substantial alignment of the two bearings, there has been employed a method in which two bearings are simultaneously pressed into the housing after a correcting pin is inserted into the housing. In the case of a bearing having hydrodynamic pressure generating grooves formed in the bearing surface, if forcible correction is made by using the correction pin, this will result in the correction pin cutting into the hydrodynamic pressure generating grooves in the bearing surface to collapse said grooves, making it impossible to obtain a stabilized hydrodynamic effect. On the other hand, the operation of press-fitting without using the correction pin will fail to provide the necessary alignment between the bearings. Further, Japanese Patent Kokai Heisei 2-107705 discloses an arrangement in which two bearing surfaces are formed in axially spaced from each other and in which a region between the bearing surfaces has a greater diameter than that of the bearing surfaces. This arrangement, though free from the aforesaid problems in practice, cannot prevent the unstable vibrations, such as whirl, because of the lack of hydrodynamic pressure generating grooves in the bearing surfaces.
As for a method of forming hydrodynamic pressure generating grooves in bearing surfaces, such a method has been a known that comprises the steps of inserting into an inner peripheral surface of a bearing blank a shaft-like jig which holds a plurality of circumferencially equispaced balls harder than the bearing blank, imparting a spiral movement to the balls through the rotation and advance of the jig while pressing the balls against the inner peripheral surface of the blank, thereby to plastically work a region of hydrodynamic pressure generating grooves method of, which method (Japanese Patent 2541208). In this method, the blank bulges in a region adjacent the hydrodynamic pressure generating grooves during forming, and such bulge has to be removed as by lathing or reaming (Japanese Patent Kokai Heisei 8-232958). For this reason, the number of manufacturing steps increases. Further, a driving mechanism and an advancing mechanism for the jig are required, thus complicating the manufacturing equipment.